Laser-markable plastics

ABSTRACT

The present invention relates to laser-markable plastics of which a feature is that they include, as absorber material, a mixture of pearl luster pigments and/or non-lustrous metal oxide-coated mica pigments and inorganic platelet-form substrates.

[0001] The present invention relates to laser-markable plastics of whicha feature is that they include, as absorber material, a mixture of pearllustre pigments and/or non-lustrous metal oxide-coated mica pigments andinorganic platelet-form substrates.

[0002] The labelling of production goods is becoming increasinglyimportant in almost all sectors of industry. For example, it isfrequently necessary to apply production dates, expiry dates, barcodes,company logos, serial numbers, etc. At present, these marks arepredominantly made using conventional techniques such as printing,embossing, stamping and labelling. However, the importance ofnon-contact, high-speed and flexible marking using lasers is increasing,especially in the case of plastics. This technique makes it possible toapply graphic inscriptions, for example barcodes, at high speed even ona non-planar surface. Since the inscription is within the plasticsarticle itself, it is durable and abrasion-resistant.

[0003] Many plastics, for example polyolefins, have hitherto proved tobe very difficult or even impossible to mark by means of lasers. A CO₂laser which emits light in the infrared region at 10.6 μm produces onlya faint, barely legible mark in the case of polyolefins, even at veryhigh output levels, since the absorption coefficient of the plastics tobe processed is not high enough at these wavelengths to induce a colourchange in the polymeric material. The plastic must not completelyreflect or transmit the laser light, since if it did there would be nointeraction. However, it must also not be too high a level ofabsorption, since in this case the plastic evaporates to leave only anengraving. The absorption of the laser beams and thus the interactionwith the material depends on the chemical structure of the plastic andon the laser wavelength used. In many cases it is necessary to addappropriate additives, for example absorbers, in order to renderplastics laser-inscribable.

[0004] The article “Pearl Lustre Pigments—Characteristics and FunctionalEffects” in Speciality Chemicals, May 1982, Vol. 2, No. 2 discloses theuse of pearl lustre pigments for laser marking. Pearl lustre pigments,however, have the disadvantage that they alter very severely the colourproperties of the plastic, an effect which is often unwanted.

[0005] In DE-C-29 36 926, plastics are marked with the aid of fillerswhose colour can be altered.

[0006] DE-A 29 36 926 discloses that the inscription of a polymericmaterial by means of laser light can be achieved by adding to theplastic a filler, such as carbon black or graphite, which discolours onexposure to energetic radiation.

[0007] The fillers known for laser marking, however, have thedisadvantage either that they durably colour the plastic to beinscribed, as a result of which the laser inscription, which is usuallya dark script on a paler background, is then no longer sufficiently highin contrast—i.e. legible—or that, as for example with kaolin, themarking is very faint and only becomes readily visible when highquantities of the additive are employed.

[0008] In the laser marking of polyethylene, for example, the contrastof a marking is found to depend on the energy density of the laser, inthe sense that a higher energy density produces darker markings. Inpolypropylene, which generally gives light markings at low energydensities, somewhat darker markings can only be obtained with extremelyhigh energy densities.

[0009] Using the absorbers known from the prior art, and especially ataverage colouring lightnesses (L values of between 10 and 80), it ispossible only to obtain markings which are difficult to read, and it isimpossible to obtain, at the same time, two-colour (light and dark)markings.

[0010] The object of the present invention, therefore, was to findlaser-markable plastics which enable a two-coloured high-contrast lasermarking to be obtained on exposure to laser light and which make itpossible, given appropriate choice of the laser energy densities, toobtain light and dark markings in one colouring operation. In thiscontext, the filler or successful absorber should have a very pale,neutral inherent colour and should possess the properties of theprecoloured plastic to be marked, or should need to be employed only insmall amounts.

[0011] It has surprisingly been found that a mixture of pearl lustrepigments and/or non-lustrous metal oxide-coated mica pigments andinorganic platelet-form substrates make it possible in mediumcolourations to obtain high-contrast, high-definition and two-colouredmarkings.

[0012] The invention therefore provides laser-markable plastics,characterized in that they comprise a mixture of pearl lustre pigmentsand/or non-lustrous metal oxide-coated mica pigments and inorganicplatelet-form substrates in medium colourations.

[0013] Through the addition of this pigment mixture in concentrations offrom 0.5 to 10% by weight, based on the plastics system, preferably from1 to 5% by weight and, in particular, from 1.5 to 3% by weight, a highcontrast is achieved in laser marking. The concentration of the pigmentsin the plastic, however, is dependent on the plastics system employedand on the energy density of the CO₂ laser. The relatively lowproportion of pigment does not substantially alter the plastics systemand does not affect its ability to be processed. The mixture of pearllustre pigment and/or non-lustrous metal oxide-coated mica pigments andinorganic platelet-form substrates can be employed in virtually allconceivable proportions. Mixtures of one part of pearl lustre pigmentand 1-10 parts of inorganic platelet-form substrates, preferably 2-8parts, in particular 3-5 parts, of the inorganic platelet-formsubstrates, have proven to be particularly suitable.

[0014] Transparent plastics doped with such pigments in pure colourationmostly show a slightly metallic gleam but retain their transparency.Through the addition of from 0.2 to 10% by weight, preferably from 0.5to 3% by weight, of opaque pigments, for example titanium dioxide, thismetallic lustre can, if required, be masked completely. Moreover, it ispossible to add colour pigments to the plastics that permit colourvariations of any kind and at the same time ensure that the lasermarking is retained.

[0015] The inorganic platelet-form substrates suitable for the markingare SiO₂ flakes, phyllosilicates, such as calcine and non-calcine mica,glass, talc, kaolin or sericite, while particularly preferred micasemployed are muscovite, biotite, phlogopite, vermiculite and alsosynthetic micas. As phyllosilicate it is preferred to employ mica. Thephyllosilicates have particle sizes of 1-150 μm, preferably 5-60 μm.

[0016] All known pearl lustre pigments can be used as absorber material,as are described, for example, in the German Patents and PatentApplications 14 67 468, 19 59 998, 20 09 566, 22 14 545, 22 15 191, 2244 298, 23 12 331, 25 22 572, 31 37 808, 31 37 809, 31 51 343, 31 51354, 31 51 355, 32 11 602, 32 35 017 and 38 42 330. Particularpreference, however, is given for the use of pearl lustre pigments basedon mica flakes coated with metal oxides, especially titanium dioxideand/or iron oxide. Non-lustrous, metal oxide-coated mica pigments areknown from DE 43 40 146 and DE 19 546 058.

[0017] As absorber material it is also possible to employ a combinationof a mixture of different phyllosilicates and/or one or more pearllustre pigments.

[0018] For the laser marking it is possible to employ all knownplastics, as described, for example, in Ullmann, Vol. 15, p. 457 etseq., Verlag VCH. Examples of suitable plastics are polyethylene (PE),polypropylene (PP), polyesters, polyphenylene oxide, polyacetal,polybutylene terephthalate, polymethyl methacrylate, polyvinyl acetal,acrylonitrile-butadiene-styrene (ABS), acrylonitrile-styrene-acrylate(ASA), polycarbonate, polyether sulfone, polyether ketones and theircopolymers and/or mixtures thereof. Polyolefins are particularlysuitable owing to their mechanical properties and the inexpensiveprocessing techniques.

[0019] Preference is given to the use of PE-ED, PE-LD, PE-LLD and PP andalso copolymers of PE and of PP.

[0020] The pigment mixture is incorporated into the plastic by mixingthe plastics granules with the phyllosilicate and pearl lustre pigment.The two pigments can be added individually, simultaneously or insuccession, or else as a mixture. The pigmented plastic is then shapedunder the action of heat. If desired, adhesives, organicpolymer-compatible solvents, stabilizers and/or surfactants which aretemperature-stable under the operating conditions can be added to theplastics granules during incorporation of the pigments. The plasticsgranules/pigment mixture is generally prepared by introducing theplastics granules into an appropriate mixer, wetting them with anyadditives used, and then adding and mixing in the pigments or thepigment mixture. The resulting mixture can be processed directly in anextruder or injection-moulding machine. The mouldings produced onprocessing usually exhibit very homogeneous distribution of thepigments. The pigment can also be employed in the form of masterbatchesfor colouring thermoplastics. In this way, it is possible to meet eventhe most stringent requirements in respect of pigment dispersion.Finally, laser marking takes place, preferably with a CO₂ laser.

[0021] Inscription with the laser is carried out by introducing thesample into the beam path of a pulsed laser, preferably a CO₂ laser.Inscription with an Nd-YAG laser or with an excimer laser is alsopossible. However, the desired results can also be achieved with other,conventional types of laser which have a wavelength in a range of highabsorption by the pigment used. The shade and depth of colour obtainedare determined by the laser parameters, such as the irradiation time andirradiation output. Low energy densities lead to light markings in thepigmented plastics system, while high energy densities lead to darkmarkings. The output of the lasers used depends on the particularapplication and can readily be determined by the skilled worker in eachindividual case.

[0022] The novel pigmented plastic can be used in all sectors wherecustomary printing processes have hitherto been employed for theinscription of plastics. For example, mouldings of the novel plastic canbe used in the electrical, electronics and motor vehicle industries. Thelabelling and inscription of, for example, cables, wires, trim strips orfunctional parts in the heating, ventilation and cooling sectors, orswitches, plugs, levers and handles which consist of the novel plastic,is possible even at difficult-to-reach points with the aid of laserlight. Owing to its low heavy-metal content, the novel plastics systemcan also be employed in packaging in the foodstuffs sector or in the toysector. The markings on packaging are notable for their resistance towiping and scratching, are stable during subsequent sterilizationprocesses, and can be applied in a hygienically pure manner in themarking process. Complete label motifs can be applied durably to thepackaging for a reusable system. Another important area of applicationfor laser inscription is that of plastic tags for the individualidentification of animals; so-called cattle tags or earmarks. The lasermarking of plastics articles or mouldings which consist of the novelplastic is therefore possible.

[0023] The examples which follow are intended to illustrate theinvention without, however, limiting it. The formulation data are givenin per cent by weight and relate to the already coloured material(plastic+pigment).

EXAMPLES Example 1

[0024] Blue-green PE-ED granules are pigmented with 0.3% Iriodin 120(TiO₂-coated mica pigment with a particle size of 5-20 μm from E. Merck,Darmstadt) and 1.5% Iriodin LS 800 (mica powder with particles<15 μm)and are processed on an injection-moulding machine. The resultingmoulding (a small plate) is subsequently inscribed using a CO₂ laser. Ata geometry of 45°/0°, the plate shows the following Lab values: L=55.3;a=−46.5; b=−12.7.

[0025] The marking with the CO₂ laser shows a distinctly pale marking atlow energy density (˜2 J/cm²) and a dark marking at an energy density of14 J/cm².

Example 2

[0026] Blue PE-HD granules which have been given a pearlescentcolouration with 0.5% Iriodin 100 Silberperl (TiO₂coated mica pigmentwith a particle size of 10-60 μm from E. Merck, Darmstadt) are injectionmoulded with the addition of 2.5% of Iriodin LS 800. The finishedmoulding can be given, depending on the energy density of the laserbeam, either a light (energy density ˜3 J/cm²) or a grey (energy density˜12 j/cm²) marking. The colour of the mouldings can be measured, at ageometry of 45°/0°, at L=40.6; a=−17.5; b=−32.6.

Comparative Example 1

[0027] Blue-green PE granules are coloured with 0.3% Iriodin 120 andthen processed on an injection-moulding machine. The resulting moulding(small plate) is subsequently inscribed with a CO₂ laser. As the energydensity (˜14 J/cm²) of the laser increases, the inscription gets darkerand darker.

Comparative Example 2

[0028] Blue-green PE granules are pigmented with 2% Iriodin LS 800 (micapowder with particles<15 μm, calcine) and are processed on aninjection-moulding machine. The marking shows a pale inscription withlittle dependency on the energy density (2-14 J/cm²).

Example 3

[0029] PP granules (PP-HD, Stamylen PPH 10 from DSM) are pigmented with0.5% Iriodin® 123, 1.5% Iriodin® LS 800 and 0.1% PV Echtblau (HoechstAG) and are processed on an injection-moulding machine.

Example 4

[0030] As Example 3, but with

[0031] 0.5% Iriodin® 123

[0032] 0.5% Iriodin LS 800 and

[0033] 0.1% PV Echtblau

Example 5

[0034] As Example 3, but with

[0035] 0.5% Iriodin® LS 810 (TiO₂ of mica)

[0036] 0.3% Iriodin® LS 800

[0037] 0.1% PV Echtblau

Example 6

[0038] PE granules (PE-HD, Hostalen GA 7260, Hoechst AG) are pigmentedwith 0.5% Iriodon® 502,

[0039] and 0.5% Iriodin® LS 800 and are processed on aninjection-moulding machine.

Example 7

[0040] As Example 6, but with

[0041] 0.5% Iriodin 502 (Fe₂O₃-coated mica pigment)

[0042] 1.0% Iriodin LS 800

Example 8

[0043] As Example 7, but with

[0044] 0.5% Iriodin® 502

[0045] 1.5% Iriodin® LS 800

[0046] The samples from Examples 3-8 were each inscribed with 4 energydensities for each formulation. Light markings were obtained at energydensities of 2.5 J/cm², 3.2 J/cm² and 7.2 J/cm² and dark markings at 9.3J/cm² and 30.8 J/cm².

[0047] The high-definition markings were distinguished by their highcontrast and were readily legible. No foaming-up of the plastics systemwas observed.

1. Laser-markable plastics, characterized in that the plastics include,as absorber material, a pigment mixture consisting of pearl lustrepigments and/or non-lustrous metal oxide-coated mica pigments andinorganic platelet-form substrates.
 2. Laser-markable plastics accordingto claim 1, characterized in that the ratio of pearl lustre pigmentand/or non-lustrous metal oxide-coated mica pigment and inorganicplatelet-form substrate in the pigment mixture is from 1:1 to 1:10. 3.Laser-markable plastics according to claim 1 or 2, characterized in thatthe inorganic platelet-form substrate is mica.
 4. Laser-markableplastics according to one of claims 1 to 3, characterized in that thepearl lustre pigment and/or the non-lustrous metal oxide-coated micapigment is a mica substrate coated with TiO₂ and/or Fe₂O₃. 5.Laser-markable plastics according to one of claims 1 to 4, characterizedin that the proportion of the pigment mixture is from 0.5 to 10% byweight, based on the plastics system.
 6. Laser-markable plasticsaccording to claim 1, characterized in that the plastic is polyethyleneor polypropylene.
 7. Laser-markable plastics according to claim 1,characterized in that they additionally comprise colour pigments.
 8. Useof the laser-markable plastics according to claim 1 as material forpreparing shaped articles which are marked with the aid of lasers,especially CO₂ lasers.
 9. Shaped articles consisting of thelaser-markable plastic according to claim 1.